I just use the Brownell's 800-grit compound that came with the lap. I've also made my own "micro lapping compound" out of the 8000-grit aluminum oxide I use with my knife strop, and some light machine oil. But that was just for lapping a couple of parts so they fit together better, not for scope rings.
Good deal...... I used too piss off folks taking so much care and time doing such for firearms that they wanted that peformed like mine. I’d tell em 30 days and they say Huh ??
That is a great looking rig BTW ! Pretty much a Mk-11 Clone per my view of the picture. Well done project !